These furnaces generally comprise a toroidal chamber and rotary hearth which is driven by a ring gear on its perimeter, or rather on the perimeter of its supporting frame, meshing with one or more pinions. This kind of drive certainly gives precise control of the movement of the hearth, which in certain types of treatment may be necessary, but the manufacturing costs are extremely high. These costs have little justification where precise control of the movement of the hearth is not required because of the methods used to process the ore.
The hearth is, as has already been stated, mounted on a supporting frame that is provided with motion-support means, such as for example a plurality of wheels acting on a circular track arranged in a plane parallel to the plane of the hearth, and guide means, such as rollers acting on at least one cylindrical track arranged around the outer or inner perimeter of the hearth supporting frame; the rollers have means for providing a thrust in a radial direction with respect to the said track.